1. Field of the Invention
This invention relates to a photometric circuit adapted for exposure control of a camera.
2. Description of the Prior Art
The conventional cameras have been generally provided with a photometric circuit which is arranged to measure light with importance set on the middle part of the angle of view of the camera, because such center preponderant photometry has a high degree of probability of giving an appositely exposed picture. Referring to FIG. 1 of the accompanying drawings, the photometric circuit of the conventional camera thus gives an output corresponding to an averaged value with the middle part having a greater weight. However, in the event of a picture plane having a SKY as an object to be photographed in a large portion thereof, a back-light object or a greatly varied brightness in part, it is often hardly possible to obtain an apposite exposure with a camera of the conventional center preponderant photometric type.
Therefore, in a known prior art photometric method for solving this problem, the picture plane is divided into a middle part and four peripheral parts which are on the upper, lower, left and right sides of the middle part; the values of light of these parts are measured independently of each other; from five light measurement values thus obtained, parameters of differences among a maximum value, an averaged value, and a center preponderant average value are computed; then, a center preponderant photometric mode, a high measured value preponderant mode, an averaging mode or a low measured value preponderant mode is selected; and automatic exposure control is carried out on the basis of a light measurement value obtained by performing light measurement in the selected photometric mode. However, this method necessitates circuit arrangement in a large scale as it requires to carry out light measurement for each of the five divided parts of a picture plane; and then to carry out a complex information processing operation on the measured values as mentioned above. This has been a drawback of this method.
Meanwhile, U.S. Pat. No. 3,884,584 has disclosed a method, wherein: A photometric element which measures light for the middle part of a picture plane and another photometric element which measures light for a peripheral part of the picture plane are series connected with each other. A series circuit thus formed is connected between an inversion input terminal and a non-inversion input terminal of an operational amplifier. A feedback loop is arranged between the output terminal and the inversion input terminal of the operational amplifier. The output of the operational amplifier is thus controlled to be of a value corresponding to a smaller one of currents produced from the two series connected elements. However, since light is measured for a middle part and a peripheral part of a picture plane which is simply divided with two photometric elements connected in series with each other in this case, if the upper half portion of the picture plane is occuplied by the sky and the lower half portion is occupied by a principal object, for example, the light of the sky is incident on these elements. In that event, the outputs of these elements depend on the intensity of the light from the sky. Therefore, in case an automatic exposure control is performed on the basis of the outputs of these elements, the principal object will have an under-exposure, thus failing to have a proper exposure.